LaBS3 revisited: a promising mid-infrared nonlinear optical material

Abstract

Thioborates are considered new potential mid-infrared nonlinear optical (NLO) crystals with wide band gaps and large second-harmonic generation (SHG) coefficients. Despite the fact that many rare-earth thioborates are structurally non-centrosymmetric, their SHG properties have only been studied very recently and their mid-infrared (Mid-IR) nonlinear optical properties have not yet been studied. In this work, we provide a new synthetic route for LaBS₃ (LBS) by solid state reactions of La₂S₃, B and S with Li₂S as flux, and we further study its thermal and optical properties, and theoretically explore the origin of its IR nonlinear optical properties by DFT calculations. Remarkably, LBS possesses balanced properties required for a good Mid-IR NLO crystal, a large SHG effect (1.2 × AgGaS₂@2.05 μm – the largest among all known ortho-thioborates), a high laser induced-damage threshold (14 times of AgGaS₂), a wide band gap (3.18 eV), a wide infrared transmission range (0.35–25 μm) and high thermal stability. Theoretical calculations suggest that its large SHG effect originates from the synergy effects of the π-conjugated [BS₃]³⁻ anions (41.82%) and the [LaS₉]¹⁵⁻ groups (58.18%).